Noise attenuating devices or "silencers" or "mufflers" have been known for some time. The devices are used in a variety of applications including mufflers for internal combustion engines and silencers for air compressors, refrigeration compressors and the like. Mufflers known in the art take a variety of forms, but generally conform to devices of the type disclosed in U.S. Pat. No. 4,930,597. Mufflers comprises an outer canister having inlet and outlet pipes. Generally the canister is divided internally into at least three chambers, including an inlet manifold chamber, an outlet manifold chamber, and a central chamber containing the noise attenuation elements. The exhaust gases from the device to be muffled enter the muffler canister and the inlet manifold. From there, the gases pass through one or more, typically two, three, or four muffler pipes to the outlet manifold. The pipes may pass directly through the central chamber of the muffler can, or may be disposed to force the exhaust gases to reverse one or more times as they pass through the central noise attenuation chamber to reach the exhaust manifold.
Typically, in muffler or silencer construction, the muffler pipes contained within the central chamber of the muffler are perforated or otherwise provided with louvres. These pipes are perforated, for either part or all of their length, with part or all of the perforated sections of the pipe being surrounded by a second outer pipe which is typically crimped or welded onto the inner perforated pipe. Exhaust gases can pass in and out of the perforations or openings in the wall of the inner pipe as they traverse the central noise attenuating chamber of the muffler from the inlet manifold to the outlet manifold. Wrapping a sheath of fibrous material around the perforated inner muffler pipe in those regions where it is surrounded by the outer sleeve pipe assists in the attenuation of high frequency noise as the exhaust gases pass through or pulse in and out of the perforated muffler pipe and into the noise-attenuating fibrous sheath. In the instance where the muffler or silencer is to be utilized on internal combustion engines or on machines having hot exhaust gases, the fibrous material making up the noise attenuating sheath must be able to withstand high temperatures. Typically in such applications, materials such as steel wool, stainless steel wool, and glass fiber have been used in muffler construction. Glass fiber is generally preferred because of its superior sound attenuating properties and low cost.
It is a common practice in the fabrication of mufflers or silencers to first produce the inner perforated muffler pipe and then wrap a fibrous mat around the perforated pipe to obtain a snug fit. Next the outer sleeve pipe is slipped over the concentric inner perforated muffler pipe and fiber sheath, and the outer pipe is crimped or welded at its ends to the inner perforated pipe.
The construction of mufflers containing concentric inner perforated pipes and outer sleeve pipes with a fibrous noise attenuating sheath between the two, presents a number of difficulties, however. For effective noise attenuation, it is desirable that the fibrous sheath fit snugly around the inner perforated pipe, with the outer sleeve pipe in turn fitting snugly about the fibrous sheath and that the fibrous sheath be reasonably densely packed to assure effective sound attenuation. The problem arises during manufacture, however, of assuring that the outer diameter of the fibrous sheath is appropriate for fitting snugly inside the outer muffler sleeve pipe. If the glass fiber sheath is of too small an outside diameter, it is a simple matter to slip the outer muffler sleeve pipe over the sheath, but the fit between the fibrous sheath and the outer muffler sleeve pipe is unacceptably loose. The wrapped fibrous sheath may partially unwind to fit against the inner surface of the outer pipe, but the resulting packing of the fibrous sheath is less dense as a result, and noise attenuation is diminished.
On the other hand, if the outer diameter of the fibrous sheath is too large after winding about the inner perforated tube, the outer sleeve pipe of the muffler cannot be slipped over the glass fiber sheath, and the workpiece must be rejected. This problem is exacerbated by the common practice in the industry of one manufacturer fabricating and shipping the inner perforated muffler pipe to a second manufacturer for the step of wrapping the pipe with the fibrous sheath, followed by re-shipment of the inner pipe and sheath assembly to the first manufacturer for affixing the outer sleeve pipe, and final assembly of the muffler. The cost of shipping the inner perforated muffler pipe from one manufacturer to the other and back, merely to use the inner pipe as a mandrel for wrapping the fibrous sheath is wasteful. Moreover, as "just-in-time" inventory control is implemented by more and more manufacturers to lower costs, the method just described causes both the muffler manufacturer and the manufacturer responsible for wrapping the fibrous sheaths to maintain unacceptably high inventories of work-in-progress. Also, the method provides no guarantee that the outer diameter of the fibrous sheath, thus made, will meet the specification required for snug fit inside the outer muffler pipe sleeve when the fiber-wrapped workpiece arrives back at the first manufacturer. Typically there is a high rate of rejection, with the consequence that the inner perforated muffler pipes must once again be shipped for wrapping with the fibrous sheath.
One attempt to solve this problem is described in U.S. Pat. No. 5,461,777 to Ikeda, et al. The patentees describe the manufacture of muffler pipe assemblies by wrapping the sound-absorbing material fibrous mat around the inner pipe, using the pipe as a mandrel. Following this step, leveling or shaving members are pressed against the rotating sheathed perforated pipe to bring the outside diameter down to the inner diameter of the outer pipe sleeve. The outer pipe sleeve is then slipped over the rotating sheathed inner perforated pipe. Subsequent rotation of the inner pipe and sheath in a direction opposite to its direction of winding causes the fibrous sheath to unwind somewhat and fill the space between the inner and outer pipes.
The need, however, still exists in the art for a more efficient and cost effective method of manufacturing muffler pipe assemblies and internal combustion engine mufflers which permits a dense packing of the fibrous noise-attenuating layer between the inner and outer pipes while solving the problem of close dimensional tolerances.